As a low-pollution coating without using an organic solvent, there has been a growing demand for a powder coating in automobile parts, electric appliances, furniture, machine tool, business equipment, toys, and the like. Coating with the powder coating thus causes low pollution, and in addition, sufficient thickness of the coating film can be achieved by one time application, and it is not necessary to apply coating as many times as in an example using a conventional solvent-type coating. That is, a time period for coating can be shortened. Moreover, as the solvent is not contained in the coating, the powder coating is advantageous also in that a pinhole will not be created in the coating film. Color variation demanded of a powder coating is also required as in the conventional coating and demands for a metallic color are also high. Among others, a metallic color similar to plating has increasingly been demanded mainly from interior products in recent years.
With the powder coating with characteristics as described above, characteristics of a coating film without containing flake pigments are excellent and no problem arises. On the other hand, if flake pigments are contained, problems as follows will arise.
For metallic coating with a powder coating, conventionally, a melt blending method in which flake pigments are sufficiently mixed and kneaded with a resin or coloring pigments in advance using a melting method, and thereafter a resultant substance is pulverized by crushing or the like; a dry blending method in which resin powders and flake pigments are mixed and applied, a bonded method using resin powders having a surface adhering to a flake pigment, and the like have been known (Japanese Patent Laying-Open No. 51-137725 (Patent Literature 1), Japanese Patent Laying-Open No. 09-071734 (Patent Literature 2), Japanese Patent Laying-Open No. 50-034642 (Patent Literature 3), WO2002/094950 (Patent Literature 4), and the like).
In the melt blending method, the flake pigments tend to deform during the step of mixing and kneading or the subsequent step of controlling a resin powder particle size by crushing, and excellent metallic feel cannot be obtained after coating. In addition, when aluminum flakes are used as the flake pigments, an active surface of aluminum is exposed in the crushing step and possibility of ignition, dust explosion or the like will be higher.
In the dry blending method, deformation of flake pigments is relatively unlikely. On the other hand, as the pigments need to bear electrical charges during coating, a surface thereof should be coated with a resin or the like in advance, if metal pigments such as aluminum flakes are used. Unless coating with a resin coat or the like is provided, an applied voltage may leak to cause voltage lowering or a spark may be produced in the vicinity of an electrode particularly in corona-type electrostatic coating. In particular, the latter may become a source of ignition of dust explosion, and hence coating with substantially uncoated metal pigments (flake pigments) with the use of the dry blending method may be considered as impossible. In addition, even in a case of such a flake pigment that a surface of a metal pigment is coated with a resin, as the flake pigment and the resin powder have different charged rates respectively, a separation phenomenon between the resin powder and the flake pigment takes place during coating, resulting in lower performance in terms of design of the coating film. Further, a ratio of content of the pigments varies before and after application of the powder coating. Therefore, if the coating is recovered for reuse, a color tone is altered, which means that recycle of the coating is virtually impossible. Furthermore, the resin powders and the flake pigments are present in an unbalanced manner during coating, which leads to likeliness of unevenness of color.
In contrast to the two methods above, a bonded method is the best method as a form of a powder coating containing flake pigments, because the previously described problems will all be solved if a bonded structure in which resin powders and flake pigments are bonded to each other in an ideal form could be realized. How to form the bonded structure, however, is in turn an issue. A method of attaching a flake pigment to a surface of a resin powder by using a brush polisher, a method of transferring and bonding a flake pigment to a surface of a resin powder by bringing the resin powder in contact with a dispersion medium such as an alumina ball coated with the flake pigment, a method of heat-sealing a resin powder and a flake pigment to each other by dispersing resin powders and flake pigments in a three-dimensional rotary mixer or the like while a temperature around the melting point of the resin powder is set, a method of attaching a flake pigment to a surface of a resin powder with the use of a binder, and the like are known as the methods of forming a bonded structure. In particular, the method using a binder has such a feature that a high ratio of bonding between a flake pigment and a resin powder is achieved and further generation of an aggregated mass of flake pigments and resin powders is less likely in a manufacturing process, and thus it is considered as a particularly good forming method (Patent Literature 4).